1) Field of the Invention
This invention relates to an internal combustion engine provided with one or more intake ports per cylinder, each of said intake ports being capable of directing a flow of air to be inducted to a combustion chamber, and especially, to an internal combustion engine suitable for use as a stratified burning internal combustion engine in which one or more tumble flows are formed from the so-directed flow or flows of inducted air inside the combustion chamber.
2) Description of the Related Art
With an objective toward improving combustion characteristics in a combustion chamber, there are known internal combustion engines in which the configuration of each intake port is designed to direct a flow of air to be inducted into the combustion chamber.
Stratified burning internal combustion engines are also known. To permit operation at low fuel consumption, a layer of a rich air-fuel mixture is formed in a combustion chamber and this fuel-rich layer is ignited. This makes it possible to ignite an air-fuel mixture having a large air/fuel ratio as a whole. As a method for forming the layer of the rich air-fuel mixture in the combustion chamber, it is known to direct by intake ports flows of air to be inducted into the combustion chamber so that stratified swirls can be formed inside the combustion chamber. As one example of such stratified burning internal combustion engines, a stratified burning internal combustion engine in which stratified vertical tumble flows, that is, vertical vortices (hereinafter simply referred to as "tumble flows") are formed has already been commercialized as illustrated in FIGS. 37 and 38.
FIGS. 37 and 38 show the structure of one of cylinders of a 2-intake-port internal combustion engine, in which there are illustrated a cylinder block 322, a cylinder 324, a piston 326, a cylinder head 328 and a combustion chamber 330. Designated at numeral 334 is an upper wall of the combustion chamber 330. The upper wall 334 is shaped in the form of a pentroof which has inclined walls 334a,334b. Intake ports 340,342 are open through the inclined wall 334a of the combustion chamber 330 and are each provided with an intake valve 358. Incidentally, numeral 347 indicates an exhaust port arranged in communication with an exhaust passage 360, while numeral 359 designates an exhaust valve.
Intake air, which has flowed into the combustion chamber 330 through the respective intake ports 340,342, then flows along the inclined wall 334b toward an inner wall of the cylinder 324, said inner wall being located on extensions of axes of the individual intake ports 340,342, whereby tumble flows are formed in the combustion chamber 330 as indicated by arrows Fa,Fm.
As is depicted in FIG. 37, only one of the intake ports, namely, the intake port 342 is provided with an injector 312. A spark plug 310 is arranged adjacent to the intake valve 358 in the intake port 342 which is provided with the injector 312. In the vicinity of the spark plug 310, there is accordingly formed the tumble flow Fm of an air-fuel mixture which has been formed of inducted air and fuel injected from the injector 312, so that stratified tumble flows consisting of the tumble flow Fm of the air-fuel mixture and the tumble flow Fa of air are formed in the combustion chamber 330.
Even when the ratio of the air to the fuel inside the combustion chamber 330 is high, in other words, even upon lean burn in which the fuel concentration is low as a whole inside the combustion chamber 330, stable combustion is still feasible owing to the existence of an air-fuel mixture richer in fuel than those present at places remote from the spark plug 310, around the spark plug 310.
To direct the flows of air, which are to be introduced into the combustion chamber 300 through the intake ports 340,342, so that the tumble flows Fa,Fm can be formed in the combustion chamber 330, each intake port has a substantially straight configuration close to the combustion chamber 330. As the configuration of a port portion included in each intake port to perform the above-mentioned directing function, a bent configuration having an extremely small curvature can be contemplated in addition to a straight configuration. Whatever configuration is adopted for such a directing port portion, both the intake ports 340,342 have to be bent at a curvature, which is the greatest along the entire lengths of the intake ports 340,342, in the vicinity of the combustion chamber 330. For the flows of air inducted therethrough, the cross-sectional flow areas are reduced at the bent portions, resulting in such inconvenience that tumble flows cannot be obtained as desired and/or a maximum intake air quantity cannot be achieved to a desired level.